Unless your host is running some kind of special sendmail clone, then the -f command is repetitive and unnecessary. It is used to set the "FROM" field in the outgoing email, but this is set anyway within subscriber.cgi to whatever your admin address is. Really, if you specify 2 different addresses (one with -f and one within the admin screen), the address you give as your admin address will override the one you specified with -f.

I really don't know why sendmail doesn't seem to be opening. If you're getting this while trying to confirm an address you might want to try this to see if you get any other useful info:

I'm honestly not sure why you're getting that error. It sounds like it's a bit more of a system error, but it's hard to tell.

You might try adding the -faddress command as your host said, and then removing the From: lines within mailing routines within the script. It's a long-shot, but I really don't know what else to suggest.

To do that just open up subscriber.cgi, search for "sub confirm", and remove the line:
print MAIL "From: $admin\n";

On the off change this works, just search through the script for lines like:
print MAIL "From:
...and delete them.